In order to undo the placements you have excluded, the procedure I will follow will be something like this:

After selecting the appropriate Campaign, go to Networks Tab and in the bottom there is Exclusions buttons. Click on the plus sign and will get the placements which have been excluded at the ad group and campaign level. So choose the one which doesn't have to be excluded and simply delete them.

In order to undo the placements you have excluded, the procedure I will follow will be something like this:

After selecting the appropriate Campaign, go to Networks Tab and in the bottom there is Exclusions buttons. Click on the plus sign and will get the placements which have been excluded at the ad group and campaign level. So choose the one which doesn't have to be excluded and simply delete them.